These in vitro studies deal with the mechanism(s) of how mitogen and antigen-activated human lymphocytes induce cell destruction. The destructive reaction clearly involves three steps; recognition, activation of the normally quiescent lymphocyte, and the actual step which results in target cell destruction. We found that lymphocytes, when they become activated in vitro, actively secrete a soluble cell-toxin, termed "lymphotoxin", into the supernatant culture medium. There have been a number of major breakthroughs in our studies over the past year. First, isolation of a cell which is exquisitely sensitive in vitro to human LT, and the incorporation of this cell into a new, highly reproducible, and quantitative assay system, which is the most sensitive of all other assays for lymphokines presently available. The second major breakthrough is that we have found lymphotoxin secretion requires the continued presence of the activating agent. Moreover, secretion is stopped within minutes once the activator is removed. Furthermore, the regulator is probably a site on the surface of the lymphoid cell. Thus, LT can be switched on and off to induce both specific and nonspecific destruction of cells. Physical-chemical studies have revealed that low dilutions of purified LT behave in identical fashion to proliferation inhibition factor. In addition, we found that LT secreting human lymphocytes undergo clonal proliferation during the primary in vitro immune response against cellular antigens, but after clonal expansion, they are resistant to inhibitors of DNA synthesis. Finally, very exciting new observations indicate the definite role of lymphotoxin in in vitro lymphocyte mediated cytotoxic destructive reactions, and we clearly have identified LT-secreting lymphocytes in situ in rejecting human kidneys.